With increasing demand of wideband wireless services, it is a trend to aggregate more carriers to realize a higher peak rate. Meanwhile, due to the re-planning of the spectrum and the newly-emerged available spectrum, deployment of a Long Term Evolution (LTE) system on unlicensed band is under research. In the LTE system, there is a technique which aggregates multiple carriers on different frequency bands to sufficiently utilize the spectrum resources. On the unlicensed band of the LTE system, e.g., on the 5 GHz unlicensed band, there are hundreds megabytes of available frequency bands which are able to support more than 5 carriers at the same time. For another example, on the 3.5 GHz unlicensed band, at least 2 carriers may be supported at the same time. Therefore, in order to effectively utilize carriers on each frequency band, limitations of at most 5 carriers and the maximum aggregation bandwidth of 100 MHz in the existing LTE system need to be broken during the research on the carrier aggregation system.
With the increase of the numbers of downlink and/or uplink carriers capable of being aggregated in the carrier aggregation system, the addition of uplink and downlink control signaling is unavoidable. The existing design of the transmission of the uplink and downlink control signaling in the LTE system may be a bottle neck for the performance improvement of the carrier aggregation system. For example, uplink control signaling is transmitted on merely one uplink carrier. With the increase of the number of downlink carriers, the control signaling offload increases greatly, which results in performance decrease of a channel bearing the uplink control signaling, and accordingly data transmission efficiency of the uplink carrier bearing the uplink control signaling is also decreased. Therefore, it is necessary to disperse the uplink control signaling offload to other uplink carriers. For example, the uplink and downlink carriers may be divided into multiple groups. The uplink control signaling of each group is transmitted on one uplink carrier of the group. Thus, this uplink carrier may be seen as a primary carrier of the group, referred to as a primary secondary carrier. With respect to all groups, there is still a special uplink carrier, referred to as a primary carrier. In order to ensure the transmission performance of the uplink control signaling of each group, it is required to select a most reliable carrier to act as the primary secondary carrier, e.g., a carrier on the licensed band. But the carrier on the licensed band and the carrier on the unlicensed band may be unable to act as a reference for each other due to their different geographical locations or a large frequency gap, and the carriers on the licensed band are limited. Therefore, there may be a situation that the primary secondary carrier has to be selected from the unlicensed band. On the unlicensed band, since it is required to avoid interference between the LTE system and other devices working on the unlicensed band, e.g., radar or other wireless devices such as WiFi. The device based on the LTE system needs to perform a Clear Channel Assessment (CCA), i.e., the LTE device has to detect the wireless channel before transmitting uplink control signaling. The wireless channel can be occupied for transmitting uplink control signaling only when it is detected that the wireless channel is clear. If the primary secondary carrier, the uplink or downlink carrier fails to seize the channel for a long period, the carrier may be not effective, e.g., deactivated, or may be unable to provide accurate downlink synchronization reference or downlink channel measurement result. At the same time, configurations of paired uplink and downlink carriers in the existing LTE system are bound, e.g. the uplink and downlink carriers of the Pcell belong to the same serving cell, or uplink and downlink carriers of the Scell share the same secondary carrier index. Thus, the ineffectiveness of one downlink or uplink carrier generally results in ineffectiveness of the paired uplink or downlink carrier. Therefore, other carriers in the group may be unable to work normally. In addition, with the gradually development of heterogeneous network, more and more UEs may work in the carrier aggregation mode, wherein at least one pair of uplink and downlink carriers is from a macro base station, and another pair of uplink and downlink carriers is from a small base station. Generally, the UE is closer to the small base station. Therefore, the transmission performance of the uplink control signaling may be improved through feeding back the uplink control signaling to the small base station but not the macro base station which is farer. In the existing LTE system, the uplink and downlink carriers of the macro base station are bound and are selected as the primary uplink and downlink carriers at the same time. Therefore, it cannot be implemented for the UE that the downlink primary carrier is from the macro base station and the uplink primary carrier bearing the uplink control signaling belongs to the small base station.
Therefore, it is necessary to research how to configure the uplink and downlink carriers more flexibly, reduce the impact of the paired uplink and downlink carriers which are unable to work normally on normal operating carriers, and reduce the impact on other carriers.
It should be noted that, the description of the above background is merely to provide a clear and complete description for the technical solution of the present disclosure and to facilitate understand of those with ordinary skill in the art. The above technical solution should not be regarded as well-known for those with ordinary skill in the art merely because they are described in the background of the disclosure.